Prior art CRT controllers have generally used analog circuitry to generate both the control signals and the correction coil signals used to control a cathode ray tube (CRT). For example, virtually every CRT made includes a standard analog "pin cushion" circuit for modulating the CRT's vertical drive (or, sometimes, the horizontal drive) signal to compensate for the fact that different points on the CRT screen are of a different distance from the CRT's focal point.
Further, many prior art CRT's include static focus and astigmatism correction coils. These coils have an adjustable voltage or current, which is typically adjusted at manufacturing time to compensate for gun and yoke misalignment and imperfections. However, these prior art correction coils are generally used to generate a static field that does not vary with the CRT's raster screen position.
The aforementioned prior art is generally sufficient in low resolution applications to correct for gun and yoke misalignment and other gun imperfections. In such circumstances, small differences in the distances between raster lines and small astigmatisms and defocus are either not noticeable to the human eye or do not significantly affect picture quality.
In high resolution applications (typically having a resolution of at least 1000 by 1000 pixels), small imperfections in the picture generated by a CRT become more noticeable and the need for accurate picture generation makes the above mentioned imperfections more significant.
With the use of a high resolution multi-beam CRT, the inventors have found that (1) small differences in the distances between raster lines become more noticeable; and (2) "twisting" of the beams, especially at the outside corners of the display, creates noticeable imperfections in the picture generated. "Twisting" in this context means that pixels generated by parallel beams which should be vertically aligned are horizontally displaced from one another because the band of beams in the CRT is twisted by electromagnetic forces from its normal vertical orientation.
The inventors have found that the combination of using a multi-beam CRT and several different improvements over the prior art CRT systems generates the best high resolution pictures. The first improvement is the use of a vertical linearity correction circuit which adjusts the distance between each band of raster lines. A second improvement is the use of dynamic size, focus and astigmatism correction circuits which provide the ability to use different size, focus and astigmatism corrections for every "tile" on the display. A third improvement is the provision of a "detwister" correction coil and a corresponding correction circuit to compensate for twisting of the CRT beams.
All of the above mentioned improvements are implemented using digital circuitry, which allows the dynamic generation of very precise correction signals that vary as a function of screen position. Further, the use of digital correction circuitry facilitates and greatly simplifies the calibration of each separate correction signal.
It is therefore a primary object of the present invention to provide a multi-beam CRT system which dynamically corrects for the imperfections in the picture generated.
Another object of the present invention is to provide a multi-beam CRT system which corrects for twisting of the CRT beams.
Yet another object of the present invention is to provide digital correction signal generation circuitry which allows the dynamic generation of correction signals which that vary as a function of screen position.